The present invention relates to percussive and/or rotary drill tools having a pilot bit, and an eccentric reamer mounted on the pilot bit for radial extension and retraction.
Such a drill tool is described in jansson U.S. Pat. No. 4,770,259 wherein a pilot bit includes a lower drill head that is coaxial with the tool axis, and an upper threaded stem which screws into the bottom end of a guide device. The guide device is adapted to be mounted at the lower end of a drill string for rotation therewith. The pilot bit also includes an eccentric cam part situated above the drill head. In a percussive-type tool, the apparatus can be subjected to percussive impacts by a top-hole percussion mechanism, or a down-the-hole percussion mechanism, during an earth or rock boring operation.
Mounted on the eccentric cam part is a hollow reamer which is rotatable relative to the cam part, whereby relative rotation in one direction causes the reamer to be extended radially outwardly to drill a hole larger than the tool, in order to enable a hole casing to be brought down with the tool. In response to relative rotation in the opposite direction, the reamer is retracted radially inwardly to a small enough diameter to enable the tool to be withdrawn upwardly through the hole casing.
In order to stop the relative rotation at the end of a reamer-displacement operation, to retain the reamer in its extended or retracted state, the cam part and the reamer are provided with respective stop surfaces facing in opposing circumferential directions. The stop face is formed integrally on the cam part by a radially outwardly projecting lug disposed to rotate within an arc-shaped recess formed in the inner periphery of the reamer.
During a boring operation, with the reamer in an extended state, the stop surfaces of the cam part and reamer transmit the rotational cutting forces to the reamer. As a result, the surfaces are particularly susceptible to wear. When the surfaces become excessively worn, the tool must be pulled to replace the worn parts, which is costly in terms of materials and lost drilling time.
Another shortcoming involves the fact that appreciable clearances should be provided between the lug and the adjacent surfaces of the recess in which the lug moves, in order to reduce the frictional resistance to relative rotation between the cam part and the reamer. However, cuttings and other foreign matter are able to penetrate such large clearances, thereby accelerating the abrasive wearing of the surfaces.
Another drill of this general type is disclosed in British Published Application No. 2,312,700 wherein the cam part of the pilot bit carries a pin insert that projects axially forwardly and is movably received in an upwardly open, semi-annular groove formed in the reamer. The pin limits the extent of relative rotation between the cam part and the reamer by abutting the ends of the groove and also transmits rotational cutting force to the reamer. This drill is thus susceptible to wear for the same reasons as described earlier. The drill disclosed in the British application facilitates relative rotation between the cam part and the reamer by positioning a horizontal annular row of ball bearings within cooperative grooves formed in the cam part and the reamer. However, such an arrangement may permit the reamer to become cocked or skewed relative to the reamer (i.e., the axis of the reamer can become disposed non-parallel with respect to the axis of the cam part).
It would be desirable to provide an improved drilling apparatus which eliminates the above-discussed shortcomings.